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BMW Z3 Modification Help
I am installing a 3 step Shift Light, But I have some wiring Questions.

I Have done some Modifications to this 98 2.8 Roadster.
I have Added an A Pillar Dual gauge Pod and installed two Dual Microprocessor Digital Gauges,
SPA Design Oil Timp/Oil Pres, and Water Temp/Volt Meter
I ordered These Gauges from ,WWW bimmerworld.com

In fact these gauges alerted me to the fact that the Z was over heating, I was then able to replace the Water pump, thermostat, temp switch, and hose's it's now running Cooler than before. and believe it or not The Z did not do the melt down when I tried to vent the air out of the system.

I am now adding a 3 Stage Digital Shift Light, from the same maker SPA Design. I also got this at the same site.
I well be running the wire's thought the firewall into the Fuse-box then out to the (?????) wherever?
This is where I need a Guru's help.
I do not wish to damage the car by tapping into the wrong wire.

The Shift Light system has 5 wire's
Two wires to power the LED back Lighting of the instrument.
Red wire with fuse for switchable + Power
Black for Ground to Chassis
both of these I am clear on, i.e. where to tap into Power and Ground.

The remaining 3 wires
To route the signal from the Engine into the Z3
Yellow Green wire to Ground/Chassis
Black to TAC HO - nag ( usually chassis)
Red to TAC HO + plus connect to the TAC HO Output of ECU or Coil (Usually Negative side)
Here is where I'm having problems?
What is an ECU. and what does HO stand for.

If you might have the time to help out I would be grateful,
A Boy and a New Toy are always out to play and soon forgets all reasonability.
I cant believe I admitted that.